1. Field of the Invention
The present invention relates to an improvement of the retention plate used in a polishing device that mirror polishes semiconductor substrates such as silicon wafers. Particularly, by using an adhesive or vacuum adhesion and making the outer diameter of the retention plate that holds the wafer smaller than the diameter of the substrate subject to polishing, and causing the area of contact with the substrate to be reduced from the inner peripheral part side only on the outer peripheral part with a specific width, the present invention is able to reduce the polishing pressure of the outer peripheral part side, improve the polishing precision of the semiconductor substrate by delaying the polishing, eliminate the so-called roll-off of the substrate, and obtain a wafer with an extremely high flatness.
2. Description of the Related Art
At the time of manufacturing semiconductor substrates such as silicon wafers and the like, a surface treatment process such as lapping or polishing is indispensable for shaving the surface fracture layer generated at the time of slicing and obtaining a flawless single crystal face.
When grinding and polishing the substrate during the surface treatment process, in order to fix a wafer and the like to a grinding/polishing device, with the polishing device shown in FIG. 1, for example, the semiconductor substrate or wafer 3 is caused to contact the surface of the retention plate 2 held by the rotating carrier 1 and fastened using an adhesive or vacuum adhesion, the wafer 3 surface subject to polishing is caused to contact the polishing cloth 4 provided on the top surface of the polishing table 5, and the rotating carrier 1 is pressurized and rotated in order to conduct polishing.
As methods of retaining substrates subject to polishing, mainly, a method for attaching a wafer with wax to a hard ceramic or a resin board of acrylic, polycarbonate and the like for polishing bare wafers, or a method of retaining wafers by adsorption upon providing multiple perforations of minute holes or processing grooves on the retention plate surface (hereinafter called the hard chuck method) has been employed.
For the final polishing of bare wafers and polishing of surface oxide films after device preparation, mainly, a method of retaining the outer periphery of the wafer with a guide ring upon attaching the wafer with the surface tension of wafer on a soft backing pad surface (hereinafter called the soft chuck method) has been employed.
The demand for making recent semiconductor devices to be of a high precision and enlarging the diameter of silicon wafers is increasing, and polishing precision, particularly, the improvement of flatness, has been in great demand.
In the mirror polishing process that determines the final flatness of a wafer, as mirror polishing is performed upon eliminating processing distortion, materials such as urethane foam and polyester nonwoven fabric that are softer than the wafer are generally used as the polishing cloth.
When polishing is carried out with a polishing cloth made of material softer than the wafer, the polishing cloth 4 during processing becomes deformed at the outer periphery of the wafer 3, as shown in the polishing model of FIG. 2, because pressure is applied from the wafer. When polishing is carried out in this state, the outer peripheral part of the wafer 3 becomes actively processed, and the flatness of the wafer 3 after polishing deteriorates considerably at the outer peripheral part. In the drawing, the fine broken line 6 indicates the amount of polishing of the wafer, the thick broken line 7 indicates the polishing pressure, and the two point chain line 8 indicates the amount of deformation of the polishing cloth.
In the experimentation conducted by the inventor; that is, during the 15 minutes of polishing, it was confirmed that polishing progresses about 1 xcexcm in a range approximately 10 mm inward from the outer peripheral edge of the wafer in comparison with the central part thereof. Although hardening the polishing cloth and reducing the amount of deformation of the polishing cloth at the wafer periphery are considered to be effective in order to prevent a polishing abnormality known as roll-off, there is a fear of the hard polishing cloth scratching the wafer surface.
In the final polishing of wafers, adopted is a method of reducing the amount of deformation of the polishing cloth in the wafer region by establishing a retainer ring at the outer peripheral part of the backing pad and pressing down the polishing cloth at the outer peripheral part of the wafer with a retainer during polishing, or a method of reducing the pressure applied to the outer peripheral part with a backing pad of a small diameter.
However, when employing a retention plate with a small diameter, the outer peripheral part of the backing pad deforms during pressurization when a soft backing pad is used, and absorbs the difference in the processing pressure at the outer peripheral part of the wafer. Meanwhile, since the retention plate does not deform in the hard chuck method, it is difficult to acquire the effect of reducing the processing pressure at the outer peripheral part even if the diameter is made small.
Furthermore, when the diameter of the retention plate is sufficiently reduced, the pressure difference of the outer peripheral part of the retention plate applied to the wafer is directly communicated to the wafer, and depressions are formed in the parts where the pressure rapidly changes after polishing, thereby deteriorating the precision after polishing even if the roll-off is improved. Thus, a retention plate with a diameter smaller than the wafer could not be used in the hard chuck method.
The present invention has an object to provide a retention plate used in a polishing device that mirror polishes semiconductor substrates such as silicon wafers, wherein the demand for making semiconductor devices to be of a high precision and enlarging the diameter of silicon wafers is increasing, particularly a retention plate used with the hard chuck method, that can prevent roll-off, and can achieve a high improvement in flatness.
The inventor conducted various and studies aimed at the constitution of a retention plate for polishing that would not cause roll-off in the outer peripheral part of a wafer, even with a hard retention plate for polishing. That is, generally, because in wafer polishing the amount of polishing greatly depends on the polishing pressure, if the pressing force of the outer peripheral part is reduced, the above-mentioned roll-off can be reduced. Roll-off also depends on the physical properties of the polishing cloth used, but, generally, in the case of a 200 mm diameter wafer, sagging occurs in a region 10 mm from the outer periphery of the wafer, and becomes exponentially stronger towards the outer periphery of a wafer.
In causing a change of pressure so as to run counter to this roll-off shape, polishing may be carried out without holding the outer peripheral part of a wafer. Nevertheless, as mentioned above, with a hard polishing retention plate, even if the diameter of the retention plate is made smaller than that of the wafer, because the retention plate itself does not deform due to the pressing force from the wafer as with the soft backing pad, such effect is difficult to obtain. Furthermore, the fluctuations of the pressing force at the held parts and the parts that are not held become too large, and, on the contrary, the wafer shape after polishing may become worse.
Accordingly, as a result of further diligent studies concerning a retention plate for polishing that can prevent the roll-off of a wafer, it became known that, by means of using a retention plate of a constitution that gives a region in which the area of contact with the wafer is reduced due to groove processing, or a porous structure and the like, on the outer peripheral part with a diameter smaller than the wafer, a reduction of the processing pressure on the outer peripheral part becomes possible, and, because the polishing of the region where roll-off occurs is delayed, the amount removed by polishing becomes uniform within the wafer surface and processing of high precision wafer becomes possible, and this invention was thus perfected.
In other words, with respect to a retention plate for polishing which holds a semiconductor substrate on a rotating carrier for polishing use, the present invention is a retention plate used for the polishing of a semiconductor substrate in which the diameter of the retention plate is smaller than the diameter of the substrate subject to polishing, and having a means for reducing the area that contacts the substrate at the outer peripheral part thereof compared with the inner peripheral part of the retention plate.
With respect to a semiconductor wafer grinding and polishing device, the present invention makes the diameter of the wafer retention plate smaller than that of the wafer subject to processing, and, by providing a region that has caused a reduction of the area of contact with the wafer, by means of groove processing or a porous structure at the outer peripheral part of the retention plate, polishing pressure of the outer peripheral part is reduced, deformation of the polishing cloth of the outer peripheral part of the wafer during polishing is reduced, roll-off that occurs by means of this is prevented, and an extremely flat semiconductor wafer is obtained thereby.